Use estimatedAssetCount when updating collections

[kevinwo]

I noticed loading the picker becomes extremely slow (a few seconds on an iPhone 6, and as long as 10 - 15 seconds on an iPhone 4S) when you have a few thousand photos in your library. The bottleneck appears to be in calculating the number of items in a given collection – currently it requires fetching full PHAsset objects, and I think using estimatedAssetCount on the PHAssetCollection object would be a more efficient option. After implementing, I noticed the load time on the 4S comes down to 1 - 2 seconds.

[1and2papa]

@kevinwo Thank you. I indeed thought about using estimatedAssetCount in the very first beginning until I found in the document that it might not match the number in the collection:-

This count may not match the current number of assets in the collection. To get the most recent count, fetch the collection’s assets with the fetchAssetsInAssetCollection:options: method and read the count property of the fetch result.

Given that this picker supports custom assetsFetchOptions, I would also like to add a property for not using estimatedAssetCount. This PR would not be merged directly but I will add the function as said in the coming release.

[kevinwo]

Thanks @chiunam! Sounds good!

[1and2papa]

@kevinwo Just realise that estimatedAssetCount does not return the result instantly so I'll keep using fetchAssetsInAssetCollection:options: for showing the asset count of the albums.

To check whether the album is empty when updating collections, your suggestion of using estimatedAssetCount is good enough indeed so this PR will be merged. Thanks.

[kevinwo]

@chiunam Cool, thanks!